1. Field of the Invention
The present invention relates to laminated articles of generaly carbonaceous composition, and more particularly to articles such as throat inserts for rocket motors in which pyrolytic graphite is deposited on a carbonaceous substrate.
2. History of the Prior Art
It is known to make throat inserts and other rocket motor components comprising pyrolytic graphite deposited on a substrate. The pyrolytic graphite in such structures forms an ablative and insulative layer capable of withstanding temperatures on the order of 6500.degree.F. during operation of the rocket motor. The carbonaceous substrate provides a base or support for the otherwise fragile and brittle pyrolytic graphite.
Articles of this type are usually formed by heating the substrate to a relatively high temperature and depositing the pyrolytic graphite on the substrate using a hydrocarbon gas in the presence of the elevated temperature and a decreased pressure. Upon cooldown the substrate which is typically made of carbonaceous material such as graphite shrinks considerably more than the pyrolytic graphite due to substantial differences in their temperature coefficients of expansion. Due to the hollow, circular configuration of such articles in which the pyrolytic graphite forms a relatively thin layer on the inside of the substrate substantial stresses occur between the substrate and the pyrolytic graphite. These stresses which are often large enough to cause cracking of the substrate upon cooldown frequently result in delamination or other failure of the pyrolytic graphite during use of the article as a high temperature component within a rocket motor or similar device.
One approach in attempting to solve this problem involves codeposition of a rigid material with the pyrolytic graphite such that a layer is formed which varies from substantially pure pyrolytic graphite at the inner surface to the rigid material at the outer surface adjacent the substrate. The rigid material which typically comprises silicon carbide has a temperature coefficient of expansion between that of the pyrolytic graphite and that of the substrate. Consequently upon cooldown of the article following deposition of the pyrolytic graphite the rigid material shrinks more than the pyrolytic graphite but less than the substrate so as to interact with and diffuse the stresses imposed by both the pyrolytic graphite and the substrate.
While use of a rigid carbonaceous material with an intermediate temperature coefficient of expansion has met with some success for certain applications, such approach has a number of limitations making it unattractive in certain respects. For one thing the amount and temperature coefficient of expansion of the rigid material must be very carefully chosen to achieve desired results. This frequently means costly experimentation with various materials in order to determine a suitable material of correct size for a particular application. Moreover, even with careful selection of materials and other parameters, frequent failures may still result. Thus rigid, strong substrates may cause the pyrolytic graphite to break up or delaminate during cooldown without adversely affecting the substrate. Conversely, the use of weaker substrates may result in cracking of the substrates so as to ultimately result in failure of the article. Even those materials which may seem to be perfectly matched with the substrate and pyrolytic graphite may allow enough residual stress to remain in the substrate and the pyrolytic graphite so as to result in subsequent failure of the article during use.